1. Field of the Invention
This invention relates to a liquid crystal device having a regular surface alignment grating stabilising a particular liquid crystal configuration and especially to a multistable device wherein at least one stable state is a Defect state and in particular to such a liquid crystal device wherein the regular surface alignment grating has breaks, or discontinuities, therein.
2. Discussion of Prior Art
Bistable or multistable liquid crystal devices are commonly used as displays for displaying information stored in an electronic form. The devices are generally used in a pixelated form and either directly driven, matrix or actively addressed. Other uses for such devices are as light modulators, optical microwave or infrared shutters. The device may use bistability only occasionally or only partially. A typical device includes at least two electrode structures, means for applying appropriate signals and means of discriminating the two states, such as dyes, polarisers, reflectors, absorbers and illumination sources.
U.S. Pat. No. 5,357,358 describes a bistable liquid crystal device wherein both internal surfaces of the cell have a treatment giving rise to various preferred alignment directions of the liquid crystal material adjacent the surface. Careful arrangement of the surface alignment directions on opposing plates can lead to two stable states for the liquid crystal material at different azimuthal angles. U.S. Pat. No. 5,796,459 describes another surface treatment that can be used to give two stable states with different azimuthal orientation of the liquid crystal director.
Zenithally bistable liquid crystal devices are also known. U.S. Pat. No. 6,249,332 describes a liquid crystal device wherein at least one internal surface has a profile which allows the liquid crystal material to be in one of two stable states, a Continuous state or a Defect state, the two states having the same azimuthal orientation of the liquid crystal director but different zenithal orientations.
International Patent Application WO02/08825 describes how the careful design of surface profile can lead to Defect states with the defects forming close to or at predetermined features of the surface to stabilise certain configurations. In this way more than one stable state can be achieved.
WO01/40853 describes another bistable device having a surface designed to give local zenithal bistability. In this device however the profile changes over length scales of less than 15 micrometers. These changes are designed to vary the director orientations within each pixel creating micro-domains that can, for example, give scattering of incident light.
Bistable devices are also known where the combination of two suitable monostable surfaces can lead to bistability, for example as described in U.S. Pat. Nos. 4,239,345, 6,327,017 or 4,333,708.
In general bistable or multistable devices work by ensuring that the energy associated with the liquid crystal configuration is locally minimised at each stable state and that an energy barrier exists between the various stable states. In defect stabilised multistable liquid crystal devices the cell has at least one state where defects have formed. Whilst the defect represents a disclination of the liquid crystal director field and therefore some energy is contained in the distortion, the formation of defects in certain areas can result in the energy being minimised as compared to a different stable configuration without defects present or a configuration having a different arrangement of defects. This minimum may be either the global minimum of the system, or a local minimum separated from other minima by an energy barrier.
In defect stabilised devices where the surface profile provides the stabilisation of the defects the surface relief structure has a profile in one direction that has at least one concave edge and at least one convex edge. These edges act to stabilise the defects or disclinations of strength ±½. The structure is repeated across the surface to give two or more stable or meta-stable states. This is surface multistability, i.e. the stable states are produced regardless of the treatment on the other surface, although obviously the treatment on the other surface would effect the overall configuration. The repetition may be periodic or aperiodic, but with a maximum and minimum separation of the relief structures. Such surfaces are used in U.S. Pat. No. 6,249,332 and WO01/40853. Common to all prior art surface defect stabilised devices is that the at least once concave edge and at least one convex edge is parallel to the plane of the surface.
The energy barrier between the various stable states should be high enough to ensure that the correct state is selected and maintained across a range of operating conditions. It will be noted that in matrix addressed displays one pixel may have a field applied before or after it is actually being addressed to cause correct latching. Correct latching needs to be maintained across a range of operating conditions. Temperature, mechanical stress and field inhomogenities all play a part in the various latching thresholds. Consistency of latching is obviously desirable. However the energy barrier should also be low enough that the states can be selected by application of the appropriate fields. Multistable devices are often used with portable, battery driven appliances where power consumption is an issue and minimal voltage or power to latch would be advantageous.
Further it will be appreciated that the liquid crystal material is generally a continuous layer but may be divided into pixels which are separately addressable. Thus neighbouring pixels, or the liquid crystal material in the inter-pixel gap, may be latched into different stable states. The liquid crystal material at the interface is therefore subject to the elastic forces from the adjacent material which may cause partial “grow-back” or creep of the wrong, i.e. undesired, state. Some devices even have different domains within a pixel, i.e. sub-pixels having a different surface profile to other sub-pixels, to allow for latching at different thresholds to give greyscale. Again the problem of grow-back may be encountered.
Even when a device is intended to be monostable in operation it may require a particular liquid crystal stable configuration to function correctly. Change to the operating conditions could result in a change of the relative energies of states leading to growth of an unwanted state and incorrect operation.